The development of metal-free organocatalysts has emerged
as a new frontier in asymmetric catalysis
1 Since the pioneering
disclosure by List and Barbas III2 that L-proline is an effective catalyst in the intermolecular direct aldol reactions, the use of small organic molecules as catalysts has received great attentions. In recent years, L-proline and derivatives have been continuously developed for aldol and many other reactions.
3 The presence of both pyrrolidine ring and a substituent carrying at least one hydrogen bonding site are considered essential for the catalytic activities. Despite their conformity to the above rules, simple
